Q:Why should you get a distiortion pedal?
A:-Because your amp doesn't have built in distortion.
   -Because you are not satisfied with the built-in distortion of your amp.

Q:What is the difference between Distortion pedals and Overdrive pedals?
A:Overdrive is not the same as distiortion. It is meant to be added to your guitar signal in addition to your distortion pedal. With the overdrive pedal you get more sustain and compression out of your distortion. In easy words, your distortion sounds more powerfull with the addition of some overdrive.

Q:What type of distortion pedal do I need?
A: It all depends on what kind of music you want to play. There are distortion pedals targeted to every rock comunity out there. So check online for what kind of  distortion pedal your favorite guitarrist uses and you can get the same. The brand BOSS makes a good selection of distortion pedals for diferent types of music..

Q:How much should I pay for a distortion pedal?
A: Let's talk  in terms of new pedals (not used). I am a firm believer of you get what you pay for. So if you  wanna go cheap you are considerably sacrificing tone quality, not a good thing. A distortion pedal can be an essential tool of your playing that may last you a long time, so I wouldn't go cheap on that. By that I'm not saying find the most expensive one. You can get a desent/good distortion pedal from Brands like BOSS, Digitech, Ibanez, Radial, EH, etc, for around 80-150 dollars.